New legislation to control the emissions from diesel engines is being developed or put in place worldwide. As a consequence, increased efforts are underway by diesel engine designers and heavy equipment manufacturers to develop better engine control and exhaust treatment technologies. Such efforts include programs to develop diesel particulate filter (DPF) technology for trapping the particulates (e.g. soot) that are present in the exhaust streams of all diesel engines. Thus current and future legislative requirements are trending toward the adoption of particulate filtration technology to effect the removal of at least some carbonaceous particulates (e.g. soot) from the exhaust streams of diesel powered vehicles. The ceramic diesel particulate filter (DPF), which is the technology of choice for this task, filters soot out of the exhaust stream through wall-flow filtration.
A shared characteristic of current DPFs is that they accumulate soot and other particulates continuously over accumulated periods of engine operation. Continuous oxidation of the soot is intended, but not feasible over the entire engine operating range even for filter designs incorporating catalyst coatings (referred to as “catalyzed” as opposed to “uncatalyzed” filters). Therefore a necessary aspect of the use of DPF systems for diesel emissions control is that the filters must be periodically cleaned or “regenerated” once the accumulation of trapped particulates begins to adversely affect engine operation.
To effect these periodic active regenerations, the inlet temperature to the DPF is raised, typically by heating the exhaust gas entering the filter to a temperature greater than 600° C. to allow the accumulated soot to ignite and burn off. Exhaust gas heating may be accomplished, for example by the combustion of additional fuel with or without the support of an oxidation catalyst.
Within the diesel industry, filter regeneration has been recognized as problematic. The inlet conditions to the filter (exhaust gas temperature and flow, exhaust composition), as well as the soot load level of the filter, have a significant impact on the regeneration behavior of the DPF. In many cases, the soot ignition can create extreme exotherms within the filter, leading to filter cracking and/or melting and loss of filtration effectiveness. Thus significant attention has been devoted to the development of new technologies for reducing the risk of filter damage, including ways to moderate the very high filter temperatures affiliated with uncontrolled regenerations, i.e., regenerations characterized by the rapid and uncontrolled combustion of the accumulated soot.
At the same time it is desirable to complete the regeneration process quickly to reduce the fuel economy penalty and to enable nearly complete regeneration, especially under dynamic driving conditions. Quick regeneration also lowers the probability of encountering the undesired boundary condition referred to as Drop-to-Idle (DTI), during which the idling engine produces exhaust gas flows that are inadequate to effectively moderate peak filter temperatures during the soot combustion process.
Rapid regeneration can be achieved by ramping quickly to high inlet temperatures that exceed soot combustion temperatures (typically 650-750° C.). These high inlet temperatures typically cause the combustion process to progress very rapidly from the inlet of the DPF toward the filter outlet, creating the potential for excessively high temperatures near the exit face of the DPF which can damage the filter. In addition, fast heat-up regeneration can produce non-uniform and incomplete soot combustion wherein the center of the filter is regenerated but the periphery retains some level of unburned soot. This makes it difficult for engine control systems to predict what the actual filter soot loading level is and when to initiate the next filter regeneration cycle.
One strategy used to combat the potential for product failure under these circumstances is to reduce the maximum allowable soot loading level within the DPF, thereby increasing the regeneration frequency. This strategy, unfortunately, carries regeneration uniformity, regeneration efficiency, and fuel efficiency penalties, and can additionally cause high engine oil dilution levels from the excessive use of in-cylinder fuel post-injection to initiate regeneration. The latter effect reduces oil lubricity and has a negative effect on engine emissions and engine lifetime.
Engine dynamometer studies have been conducted throughout the industry under various engine operating conditions that can closely simulate the conditions encountered in actual vehicle operation under various road and off-road environments. In the course of such studies various systems for initiating soot combustion at the start of the regeneration cycle have been evaluated, including the post injection of fuel, the use of an oxidation catalyst, the use of burner fired by supplemental fuels, and the use of in-line exhaust gas heaters placed upstream of the particulate filter. All can effectively initiate filter regeneration, but none has so far been shown to be effective in managing the regeneration process after soot combustion has been initiated.
U.S. Pat. No. 5,551,971, for example, discloses a system wherein thermocouples for monitoring filter temperatures during regeneration are connected to an electronic control unit which can reduce the amount of heat energy input to the filter if filter temperatures exceed certain set points. One drawback of this approach is the need to provide thermocouples and associated circuitry directly within the exhaust system and the filter itself. Another problem is that excessive filter temperatures are known to develop from uncontrolled soot combustion within the filter whether or not the supply of supplemental heat energy to initiate that combustion is interrupted.